Operational Amplifiers (Op Amps) are widely used. A switched-capacitor network may be placed on the input to the Op Amp and clocked with non-overlapping two-phase clocks. During one phase the Op Amp is reset, clearing any stored error charge. The input is sampled and stored on a capacitor. In the other phase, the stored input is amplified by the Op Amp. The Op Amp's output is fed back to the switched capacitor network to store an error charge.
The Op Amp is actually amplifying for only one of the two phases. Since the Op Amp is not amplifying for the other phase, the Op Amp is basically performing useful work for only half of the time.
Various techniques have been devised to use this wasted time by sharing an Op Amp. A multiplexer may be placed on the input to the Op Amp, allowing two capacitor networks to input to the same Op Amp. However, a memory effect may occur on parasitic capacitances causing the two input signals to have a dependence on one another, rather than being truly independent of each other. Reset circuitry may be added to reduce this memory effect, but the reset circuitry may require additional time to operate, reducing the operating frequency of the Op Amp. The reset circuitry also adds to the size, area, cost, and parasitic load. A second pair of differential input transistors may be added, but the differential input transistors are large in size to reduce noise and increase gain. Thus adding a second differential input pair is especially costly.
What is desired is a shared Op Amp using switched capacitor networks. A shared Op Amp is desired due to the lower power dissipation, area, but without an excessive area overhead or speed loss due to the sharing circuits. A more elegant shared Op Amp is desirable.